This the same question as this:
Sql server automatically change password
However, the user never specified how he fixed it, he only said that he fixed it. I am having the exact same issue. Whenever I try to create a new user login and password, the password immediately changes to an unknown, 15-character password.
@Aram Gevorgyan was the original poster of the question
EDIT: I create a new login like this and click ok
Then if I check the properties for the login immediately after, it looks like this
I am using windows 10 and just installed the newest version of sql server express and sql server management studio